This invention pertains to a swivel chair, and more particularly to a swivel chair having a seat adjustment mechanism for adjusting the height of the chair seat.
Generally, prior art swivel chairs comprise a seat connected to a base member by an adjusting assembly, which permits the seat to be vertically moved relative to the base member. The adjusting assembly generally comprises a threaded spindle secured to the seat and supported by the base member, and a nut threaded to the spindle. In the unoccupied state, a resilient device urges the nut into locking engagement with a column member extending upwardly from the base member and into which the threaded spindle is received. In this manner, the seat and spindle may be rotated relative to the locked adjusting nut and base member to thereby vertically adjust the seat.
When the seat is occupied, the resilient device is compressed to disengage the nut from the column member so that, upon rotation of the seat and spindle, the nut rotates with the spindle to prevent any vertical adjustment of the seat.
Important to the proper operation of an adjustable swivel chair is the capability of the adjusting assembly to lockingly engage and disengage the nut with the column member in a simple, reliable, and efficient manner. In most prior art swivel chairs, the locking of the nut against rotation with the spindle is accomplished by the resilient device urging the nut, which has a groove or notch disposed therein, into locking engagement with a stop or catch radially inwardly disposed from the column member. A disadvantage in some prior art swivel chairs having this type of adjusting assembly is that the chair is rotated a full or half turn in order to properly engage the nut with the column member.
Furthermore, in those instances wherein the column member has a stop or catch radially inwardly disposed, the possibility exists for the stop or catch to be broken off or bent preventing proper engagement between the nut and the column member, thereby precluding vertical adjustment of the seat.
When these prior art swivel chairs are occupied, the resilient device is compressed to disengage the nut from the column member to allow the nut to rotate with the spindle, thereby preventing vertical adjustment of the seat upon rotation. To insure the nut rotates with the spindle, a washer having a plurality of axially extending projections thereon and keyed to the spindle, or a similar device, may be disposed between the resilient device and the nut, which has a like plurality of complementary projection-receiving grooves disposed therein. When the seat is occupied or weighted and rotated, the resilient device, washer, and nut are compressed together so that the projections are lockingly received within the grooves, thereby insuring the nut rotates with the spindle. However, when this swivel chair is unoccupied and the seat rotated for vertical adjustment, the nut and the washer slidably rotate relative to each other causing an undesirable bumping or jarring effect because of the projections rotatably engaging and disengaging the grooves in the nut.
Another disadvantage, is the eventual wearing down of the projections or grooves to such an extent that, upon the seat being occupied or weighted, the nut may not properly lock for rotation with the spindle.
In other types of prior art swivel chairs, the resilient device may be found to be disposed between the nut and the portion of the column member with which the nut is lockingly engageable. Various means or devices are provided for locking the nut to the column member, however, the undesirable possibility exists that the means or device may become hung up or engaged by the resilient device, thereby preventing vertical adjustment of the chair.
A problem which can be encountered in chair height adjustment mechanisms of the general type discussed above is the tendency of the adjustment nut to rotate relative to the spindle when the chair is occupied and when no adjustment in the chair height is desired. When using a Teflon bearing, for example, a "slip-stick" phenomenon occurs wherein the coefficient of friction between the metal adjusting nut or metal bearing washer and the Teflon bearing is larger under static conditions than it is when the two elements are moving relative to each other as the chair is rotated. If the static frictional forces are sufficiently great, they may be higher than the forces between the adjusting nut and spindle so that the nut rotates relative to the spindle when the chair is occupied and results in an undesirable change in the height of the chair. Increasing the friction between the nut and spindle at all times, however, is not desirable because of the desirability of the nut rotating freely relative to the spindle while the chair height is being adjusted.